1. Field of the Invention
The present invention relates to an image forming apparatus formed in a manner that toner image is attracted directly on a recording sheet or a plane paper without utilizing a photosensitive drum or other latent image carrier. Embodiments of the invention relate to an image forming apparatus having a toner carrier and a backing electrode on which surface the recording sheet is movable, wherein the backing electrode is disposed to face to the toner carrier, and having means for toner passage disposed therebetween, wherein the means for toner passage is consisted in a plurality of toner slots effectable to open and close electro-magnetically arrayed along desired direction.
2. Description of the Prior Art
A well known electro-photograph apparatus, which is used as printers and copying machines, operates by a process involving imprinting an electrostatic latent image on a photosensitive drum, developing the electrostatic latent image by applying a developing agent of single or two components, and transferring the visible developed image to a record surface such as a sheet of paper.
As far as the electrophotographic apparatus is based on the process of transferring the toner image on the recording sheet by means of the photosensitive drum, it involves, beside the photo-sensitive drum for carrying the electrostatic latent image, means for imprinting the latent image on the drum, means for transferring the toner image borne on the drum, means for cleaning the residual toner particles attached on the drum, means for erasing the latent image, and means for recharging such as a corona discharge device, resulting in the apparatus being complicated, especially when a number of such means must be disposed around the drum. Also, the resulting apparatus typically must be extraordinarily large in size in order to place such means around the drum so that the apparatus may perform its function.
An image forming apparatus to overcome drawbacks discussed above is disclosed in the U.S. Pat. Nos. 4,478,510 and 4,491,855 and U.K. Pat. Application GB 2 108 432A (referred as `the first prior technique` hereinafter). The first prior technique employs means for toner passage, arrayed with one or two rows of toner slots effectable to open or close electromagnetically, arranged orthogonally (of which direction is referred as `scanning direction` hereinafter) to the direction of the movement of a recording member movable on a backing electrode (of which direction is referred as `subscanning direction` hereinafter), disposed between the backing electrode and a development roller which carries toner, in which a signal voltage corresponding to image information is applied to the means for toner passage to open selectively the toner slots during the movement of the recording member on the backing electrode, to transfer the toner borne on the surface of the toner carrier to the recording member through the toner slots.
The first prior technique, however, includes some restrictions for finer resolution (or printing dots density) which requires a more dense array of the slots, which issues other mechanical and manufacturing tasks.
A technique to resolve drawbacks as discussed above is disclosed in the PCT/SE88/00653 or International Publication No. WO89/05231 (referred as the basic technique hereinafter).
FIGS. 7(a) and 7(b) of the drawings show a principle of the basic technique which comprises a toner carrier 1 including a sleeve-like development roller la borne electromagnetically toner in a thin layer thereon, a control grid 3 arranged in a matrix and disposed between the toner carrier 1 and a backing electrode 2 to oppose thereof, wherein the control grid 3 is applied with a control voltage in direction X-Y axes to form a developing electric field corresponding to an image information, which subjects selectively toner slots 3a in the control grid 3 effectably to open and close thereof, causing the transfer of toner corresponding to the image information through the toner slot 3a in the control grid 3 on the recording paper 4 arranged on the surface of the backing electrode 2. Though the basic technique resembles to the first prior art in the view point above, the basic technique especially forms the control grid 3, as shown in FIG. 8(a), in a matrix of a plurality of X-axis loop wires X1-X2, . . . of which paired wires are formed in a loop arranged in a direction of the main scanning direction (X), and of a plurality of Y-axis paired loop wires Ya1-Ya2, . . . , wherein the two groups of the paired loop wires are arranged to intersect each other in an angle other than right angle, forming the portions surrounded by the paired loop wires Ya1-Ya2 on Y-axis and X1-X2 on X-axis as the toner slots 3a.
An electric voltage is applied to control grid 3, as shown in FIG. 8(b), successively on wires X1-X2, . . . with a time lag corresponding to the movement of the recording paper 4, resulting in a printing dot pattern passing through the toner slots 3a in a raw formable the dot pattern dense without arranging the interval between the wires Ya1-Ya2, . . . , nor the interval of toner slots 3a shorter in the main scanning direction.
There reveals, however, a lot of issues to realize a practical use in the first prior art technique and the basic technique as well.
The first issue involves clogging of the toner slot, and fade printing due to the sticking of residual toner.
As the basic technique is so constructed as to formed the dot pattern on the recording paper corresponding to the image information passing the toner through the toner slot, and as the control grid 3 is coated with an insulating layer thereon to prevent an electric leakage to the toner carrier 1 etc., toner sticks to the inside surface of the toner slot 3a electrostatically due to the friction between the toner and the toner slot 3a circumference or the surface of the control grid 3, liable to disturb a free flow of toner, and liable to disorder the image pattern formed on the recording paper 4.
What's worse, the clogging due to the sticking of toner on the toner slot 3a becomes more apparent, if the toner slot 3a was made to smaller to obtain the finer resolution of dot pattern.
In the event when a toner container is changed to a desired color, the toner adheres on the control grid spoils a proper coloring.
The toner passed through the toner slot 3a does not only clog therein, but also adheres circumferentially at a part of the toner slot 3a faced to the backing electrode 2. The toner at the port drops toward the backing electrode 2, and scatters around the print dots to fade a boundary line and to reduce the resolution.
The second issue is the prevention of electric leakage from the control grid 3 to other members.
The basic technique, as described above, has to dispose the toner carrier 1 close to the backing electrode 2 with a narrow space, a few hundreds micron meters for example, to form the electric field for development, and further, has to dispose the control grid 3 and the recording paper 4 therebetween, yielding some allowance for the thickness distribution of recording paper 4 in an interspace between the control grid 3 and the backing electrode 2, and yielding the thickness of the toner layer 90 in a space between the toner carrier 1 and the control grid 3, resulting the spaces extraordinarily tight, i.e., the space between the control grid 3 and the toner carrier 1, or the toner layer 90, and the interspace between the control grid 3 and the recording paper 4.
As the control voltage, as described above, is applied between the toner carrier 1 and the control grid 3 space with said tight space, there happens the discharge or leak between the toner carrier 1 and the control grid 3 to destroy the formation of the image, in the event when dielectric strength reduces due to the higher humidity; and also toner makes a bridge between them as an electric conductive member resulting in a failure to form the image, when the toner is made of electrically conductive materials.
As the developing voltage is applied between the toner carrier 1 and the backing electrode 2, and as the control voltage is applied on the control grid 3 in the tight space, the image fails to form due to the discharge or leak when the dielectric strength reduces due to the higher humidity, or in an occasion when a wet recording sheet is introduced in error therebetween.
A deviation of the interspace between the control grid 3 and the recording sheet 4 modifies the dot size on the recording sheet 4 through the toner slot 3a failing to form a fine image.
Although it is desirable to keep the interspace accurate, the recording sheet 4 floats and meanders. If one attempts to hold the recording sheet 4 in place by contacting the recording sheet 4 with means for positioning then it is likely that the image will be impaired, as the toner is not fixed yet on the recording sheet 4.
The third issue of the basic technique relates to the intensity of the print dot being under the influence of the control voltage applied on the control grid 3.
Further to the above, as will be described below, the open voltage -V.sub.1 for effecting to open electromagnetically the toner slot 3a has always the same polarity of the charged particles, thus the passage of the toner slot 3a formed by the open voltage -V.sub.1 becomes geometrically smaller; and that, the smaller the size of the toner slot 3a is made to obtain finer resolution, the narrower the physical area of the passage becomes, resulting in a thin dot, or forming image with poor intensity.
The fourth issue is a toner stain in a marginal area or a region where no print is developed.
In the development roller of the basic technique acting as the toner carrier 1, the toner layer on the development roller is formed uniformly with a doctor blade in more wider than the maximum width of the printing space to make the formation of the image easier.
Thus, as shown in FIG. 10, the control grid 3 formed with meshes and disposed in the space between the development roller 1 and the backing electrode 2 is extended beyond the maximum space for printing space to the marginal area or the region 40 where no print is provided. In the marginal area 40, as no toner transfers toward the backing electrode 2, the toner forms continuously a brush on the development roller 1 up to a grown-up toner layer 90, to pile a molehill on the control grid 3, to drop finally on the backing electrode 2, and to produce a stain thereon, even if the toner slot 3a has been electromagnetically effected to close.